There's a very big "if" that's missing from the above. Genesis allows for more Rejuvenation casts if (and only if) all available targets have Rejuvenation already. Unless all targets have Rejuvenation, you'd do more healing if you simply spent that GCD casting Rejuvenation on someone. You can't keep ten targets perma-blanketed while also keeping WG/Harmony/Lifebloom up, so Genesis is only a healing increase in a five man group or if your raid is split up.
The spell can be worth casting in other scenarios too, but in a raid setting you'll be sacrificing long term HPS to get short term HPS. The spell will probably see more use in five mans than in raids.